middle-end: Teach CSE to be able to do vector extracts.

This patch gets CSE to re-use constants already inside a vector rather than
re-materializing the constant again.

Basically consider the following case:

#include <stdint.h>
#include <arm_neon.h>

uint64_t
test (uint64_t a, uint64x2_t b, uint64x2_t* rt)
{
  uint64_t arr[2] = { 0x0942430810234076UL, 0x0942430810234076UL};
  uint64_t res = a | arr[0];
  uint64x2_t val = vld1q_u64 (arr);
  *rt = vaddq_u64 (val, b);
  return res;
}

The actual behavior is inconsequential however notice that the same constants
are used in the vector (arr and later val) and in the calculation of res.

The code we generate for this however is quite sub-optimal:

test:
        adrp    x2, .LC0
        sub     sp, sp, #16
        ldr     q1, [x2, #:lo12:.LC0]
        mov     x2, 16502
        movk    x2, 0x1023, lsl 16
        movk    x2, 0x4308, lsl 32
        add     v1.2d, v1.2d, v0.2d
        movk    x2, 0x942, lsl 48
        orr     x0, x0, x2
        str     q1, [x1]
        add     sp, sp, 16
        ret
.LC0:
        .xword  667169396713799798
        .xword  667169396713799798

Essentially we materialize the same constant twice.  The reason for this is
because the front-end lowers the constant extracted from arr[0] quite early on.
If you look into the result of fre you'll find

  <bb 2> :
  arr[0] = 667169396713799798;
  arr[1] = 667169396713799798;
  res_7 = a_6(D) | 667169396713799798;
  _16 = __builtin_aarch64_ld1v2di (&arr);
  _17 = VIEW_CONVERT_EXPR<uint64x2_t>(_16);
  _11 = b_10(D) + _17;
  *rt_12(D) = _11;
  arr ={v} {CLOBBER};
  return res_7;

Which makes sense for further optimization.  However come expand time if the
constant isn't representable in the target arch it will be assigned to a
register again.

(insn 8 5 9 2 (set (reg:V2DI 99)
        (const_vector:V2DI [
                (const_int 667169396713799798 [0x942430810234076]) repeated x2
            ])) "cse.c":7:12 -1
     (nil))
...
(insn 14 13 15 2 (set (reg:DI 103)
        (const_int 667169396713799798 [0x942430810234076])) "cse.c":8:12 -1
     (nil))
(insn 15 14 16 2 (set (reg:DI 102 [ res ])
        (ior:DI (reg/v:DI 96 [ a ])
            (reg:DI 103))) "cse.c":8:12 -1
     (nil))

And since it's out of the immediate range of the scalar instruction used
combine won't be able to do anything here.

This will then trigger the re-materialization of the constant twice.

To fix this this patch extends CSE to be able to generate an extract for a
constant from another vector, or to make a vector for a constant by duplicating
another constant.

Whether this transformation is done or not depends entirely on the costing for
the target for the different constants and operations.

I Initially also investigated doing this in PRE, but PRE requires at least 2 BB
to work and does not currently have any way to remove redundancies within a
single BB and it did not look easy to support.

gcc/ChangeLog:

	* cse.c (add_to_set): New.
	(find_sets_in_insn): Register constants in sets.
	(canonicalize_insn): Use auto_vec instead.
	(cse_insn): Try materializing using vec_dup.
	* rtl.h (simplify_context::simplify_gen_vec_select,
	simplify_gen_vec_select): New.
	* simplify-rtx.c (simplify_context::simplify_gen_vec_select): New.

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